> 19xx I read as a date in the range 1900..1999 but with readability, resp. precision, limited to a 100 year span
Could you please clarify that? Does it depends on the context? What do 
you mean 19xx would be within a single-choice list vs. an inclusive 

As of today's specification, x's are unambigously allowed only as the last 
digit of the year within inclusive lists of multiple years. I wonder what 
- in such a context - would be the semantic difference between




If there is such a semantic difference, I suggest to clarify it in the 

> {1957..1958, 1960-1969, 1971}
I assume you mean

{1957..1958, 1960..1969, 1971}

> So 1x90 would mean: the years 1190, 1290, 1390, 1490, 1590, 1690, 1790,1890, 1990
And 1090, too.

> What would be a use case for this?
I do not know. Obviously, 1x90 is much shorter to write, but if there are 
no use cases, we could remove it.

On the other hand, an x as a last digit of a year does not significantly 
shorten the expression.

> > 2011-06-xxT12:00:00
> I would not want to pursue this beyond the single 'x' at the end of a year.
My guess is that


would be more useful than


because the latter can easily be replaced by


whereas the former could only be replaced by a quite lengthy 30-element 
list such as

{2011-06-01T12:00:00, 2011-06-02T12:00:00, 2011-06-03T12:00:00, etc. }

... if inclusive lists with anything other than years were allowed, which 
the specification does not exemplify nor exclude.